Method and Apparatus for Increasing a Perceived Resolution of a Display

ABSTRACT

According to one embodiment, a method of increasing a perceived resolution of a display includes directing light at a optical dithering element and repeatedly transitioning the optical dithering element from a first position to a second position and then back to the first position such that the mirror alternately reflects light to a first position on the display and then to a second position on the display. Each transition of the mirror includes controlling any overshoot or ringing in the position of the optical dithering element by providing a predetermined drive signal to the optical dithering element to smoothly accelerate and decelerate the element during the traverse between the first and second positions.

TECHNICAL FIELD OF THE INVENTION

This invention relates generally to display systems and moreparticularly to a method and system for increasing a perceivedresolution of a display.

BACKGROUND OF THE INVENTION

Televisions and other types of displays are pervasive in today'ssociety. Recent years have seen the introduction of higher definitiondisplays. Engineers continue to try to increase the resolution ofdisplays to provide better picture quality, but also face constraintsassociated with providing such increased resolution.

One approach for increasing the resolution of a display involvesincreasing a perceived resolution of a display by a user. Rather thanproviding more pixels, a first image is displayed including a set numberof pixels corresponding to the same number of sample data points of theimage to be displayed. Then at a time period very close to the displayof the first image, a second image is displayed including the samenumber of pixels but with slightly different sample points of the image.This second image on the display is offset by a small amount from thedisplay of the first image. The human eye perceives both images as beingdisplayed at the same time, resulting in an effective doubling of thedisplay resolution. This technique is referred to in the industry bymany names including modulation, optical dithering, and SmoothPicture™.

In one technique for effecting the offset of the two images, a mirror isused as an optical dithering element to direct light corresponding topixels to be displayed onto the display. The mirror is repeatedlyswitched from one position to another such that the first position ofthe mirror corresponds to a display of an unshifted image and the secondposition corresponds to a display of a shifted image. Thus rapidpositioning of the mirror between the first and second positions allowsan increase in the perceived resolution of the display.

In order to control the position of an optical dithering element, it isuseful to measure the position of the optical dithering element.However, such a measurement can often be costly, adding undue expense tothe underlying product.

SUMMARY OF THE INVENTION

According to one embodiment, a method of increasing a perceivedresolution of a display includes directing light at a optical ditheringelement and repeatedly transitioning the optical dithering element froma first position to a second position and then back to the firstposition such that the mirror alternately reflects light to a firstposition on the display and then to a second position on the display.Each transition of the mirror includes controlling overshoot and ringingin the position of the optical dithering element by providing apredetermined control signal to the optical dithering element. In oneembodiment, this allows smooth acceleration and deceleration of theelement during the traverse between the first and second positions.

Some embodiments of the invention provide numerous technical advantages.Some embodiments may benefit from some, none, or all of theseadvantages. For example, according to one embodiment, a desirableposition versus time characteristic of an optical dithering element isachieved without the use of expensive servo control feedback loops. Thisallows for optical dithering in a cost effective manner.

Other technical advantages may be readily ascertainable by one of skillin the art.

BRIEF DESCRIPTION OF THE FIGURES

For a more complete understanding of the invention, and for furtherfeatures and advantages, reference is now made to the followingdescription, taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a schematic diagram illustrating a system for displaying lightwith increased perceived resolution according to the teachings of theinvention;

FIG. 2A is a graph illustrating a desirable transition of the mirror ofFIG. 1 from a first position to a second position;

FIG. 2B is a graph illustrating a position versus response curve for themirror of FIG. 1 during transition without a control signal controllingthe position response of the mirror;

FIG. 2C is a graph illustrating the position of the mirror with respectto time for numerous transitions of the mirror between first and secondpositions without use of a control signal to control the response;

FIG. 3A is a graph illustrating a desirable response, a control signalused to develop a desirable response, and an undesirable response of thesystem of FIG. 1;

FIG. 3B is a graph analogous to FIG. 3A but illustrates several periodsof responses and associated control signals for the position of themirror of FIG. 1;

FIG. 3C is a graph illustrating overshoot versus a plurality ofparameters that may be used to determine a desirable control signal forthe system of FIG. 1;

FIG. 4A is a block diagram of the dither control architecture accordingto one embodiment of the invention;

FIG. 4B is a block diagram of the field programmable gate arraycontroller of FIG. 4A, according to one embodiment of the invention;

FIG. 4C is a block diagram of the dither control architecture accordingto a second embodiment of the invention;

FIG. 4D is a block diagram of an application specific integrated circuitcontroller of FIG. 4C;

FIG. 5 is a graph illustrating sampling of a plurality of data pointsfor use in control of an optical dithering element according to theteachings of the invention;

FIG. 6 is a flowchart illustrating a method for controlling an opticaldithering element according to the teachings of the invention;

FIG. 7A is an error signature map corresponding to the drive waveformhaving a low amplitude;

FIG. 7B is an error signature map having the drive waveform of a desiredamplitude;

FIG. 7C is an error signature map corresponding to the drive waveformhaving a high amplitude;

FIG. 8 is a table illustrating a plurality of error signatures and thecorresponding adjustment to delay width and magnitude of the drivesignal;

FIG. 9A is a schematic diagram illustrating an optical dithering elementand an associated position measurement system according to the teachingsof the invention;

FIG. 9B is a schematic diagram along the lines 9B-9B of FIG. 9A showingadditional detail of the aperture and the device arm of FIG. 9A;

FIG. 9C is a graph illustrating the photocurrent versus distance for thephotointerrupter of FIG. 9A;

FIG. 10 is a circuit diagram illustrating control circuitry and theassociated photointerrupter of FIG. 9A; and

FIG. 11 is a flow chart illustrating a method for providing positionfeedback for an optical dithering element.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention and its advantages are best understood byreferring to FIGS. 1-11 of the drawings, like numerals being used forlike and corresponding parts of the various drawings.

FIG. 1 is a schematic diagram illustrating a system 10 for displaying animage with increased perceived resolution. System 10 includes an imagesource 12, an optical dithering system 14, and a display 16. Imagesource 12 is operable to generate light 18 representative of an imagefor eventual display on display 16. According to one embodiment, imagesource 12 comprises a digital micro-mirror device (DMD), available fromTexas Instruments, for selectively modulating light to represent animage. In one embodiment, image source 12 may sequentially generatelight of different colors and provide those different colors in sequencefor display on display 16. Image source 12 may provide these differentcolors for appropriate time periods such that a user's eye viewing thelight on display 16 will integrate the various colors to result in adesired color to be displayed. According to one embodiment in whichimage source 12 is a DMD, the number of mirrors in the DMD is equal tothe unenhanced resolution of the display of display 16. Thus imagesource 12 may provide an array of light signals 18 for eventual displayon display 16.

Optical dithering system 14 receives light signals 18 and reflects themonto display 16. Optical dithering system 14 includes an opticaldithering element 20, which may be a lens, mirror, or other deviceoperable to selectively direct light to a desired location. In thepresent example, optical dithering element 20 is a mirror. Mirror 20rotates about an angle 22 between first and second positions toselectively reflect light 18 from image source 12 to display 16 into oneof two positions. In one embodiment, angle 22 is very small and on theorder of 0.015 degrees. This corresponds to approximately four micronsof vertical movement of the end of mirror 20. Thus, mirror 20 producesan offset light beam 24 and an unoffset light beam 26 for display ondisplay 16. If mirror 20 is rotated sufficiently rapidly between thesetwo positions, display 16 appears to have a resolution equal to twicethe unenhanced resolution. It should be noted that the perceivedresolution of display 16 could be increased by a factor of four, ratherthan two, if mirror 20 is rotated about two axes rather than just oneaxis. The teachings of the invention may be incorporated into such asystem as well.

Mirror 20 rotates about a pivot point 28. An actuator 30, which in oneembodiment is a voice coil, either pushes or pulls mirror 20 up or downto effect movement between the first and second positions. Actuator 30may take other forms that are operable to effect movement of mirror 20.A spring 32 is coupled to mirror 20 such that the resulting movement ofmirror 20 is approximately proportional to the force applied by actuator30. By utilizing spring 32, which provides an approximately proportionalresponse between a force applied and position of the spring, control ofthe position of mirror 20 is facilitated.

A controller 34 is provided to control actuator 30 such that mirror 20is rotated between first and second positions in a desirable manner.Controller 34 communicates with actuator 30 over line 38. Controller 34may take any suitable form, such as an ASIC or an FPGA, and may beprogrammed according to the teachings of the invention as described ingreater detail below.

A position sensor 40 provides an indication of the position of mirror20. Feedback from position sensor 40 may be provided to controller 34;however, as described in greater detail below, feedback of the positionof mirror 20 has a limited role in one embodiment. Any suitable positionsensor may be used that provides an indication of the position of mirror20; however, according to one embodiment, the position sensor describedin FIGS. 9A through 11 is utilized.

One challenge recognized by the invention with transitioning mirror 20from a first position to a second position involves overshoot andringing of the position of mirror 20. Actuator 30 may apply a force tomirror 20 to cause it to start moving from a first position to a secondposition (or between multiple positions in other embodiments), butcausing mirror 20 to stop at the second position requires some control.One approach for solving this problem would be to provide a positionfeedback signal such that a servo control loop could precisely controlthe position of mirror 20 and it could transition from a first positionto a second position with minimal overshoot. However, such a controlsystem would require a high bandwidth feedback and a very rapid controlsystem, both of which would significantly increase costs associated withmirror system 14.

According to the teachings of the invention, rather than providing acontrol feedback loop and precisely controlling the position of mirror20 during its transition from a first position to a second position, apredetermined control waveform is transmitted from controller 34 toactuator 30 over line 38 that effects a desirable position versus timeresponse of mirror 20 during transition from its first position to itssecond position and then back to its first position. By utilizing apredetermined waveform, lower cost components may be used and expensiveand high bandwidth feedback control systems are not required. Additionaldetails of such a control signal are described in greater detail belowin conjunction with FIGS. 2A through 3C.

FIG. 2A is a series of graphs illustrating the transition of mirror 20from a first position to a second position. The top graph of FIG. 2Aillustrates the position of mirror 20 as mirror 20 transitions from afirst position to a second position, and includes curves 50 and 52. Thebottom graph is an indication of a particular color of light 18 providedby image source 12 for a given pixel that is being displayed at aparticular time. It should be noted that image source 12 generates sucha signal for a large number of pixels. According to one aspect of theinvention, it has been determined that transitioning mirror 20 from afirst position to a second position is desirable to be performed in atime period in which only blue light, if any, is displayed. As describedabove, in one embodiment, light 18 is provided sequentially in variouscolors and the user's eye integrates these colors to generate a desiredcolor to be displayed. The proportion of the time frame in which anyparticular color may be transmitted determines the resulting color.Light 18 includes, in this embodiment, white light 56, blue light 58,red light 60, and green light 62. Thus, in the time period in which bluelight 58 may be displayed the teachings of the invention recognize thatmirror 20 should be transitioned during this time period. This isbecause transitioning while blue light is displayed is less perceptibleto a viewer's eye than transitioning while other colors are displayedbecause the human eye has its lowest spatial response in blue light andblue light produces the fewest lumens. It should be noted that althoughtransitioning normally occurs during this time period it is notnecessarily the case that blue light is displayed. Rather, this timeperiod is the time period in which no color other than blue may bedisplayed. Transitioning may also occur during the time period dedicatedto transmitting other colors of light, if any, that are determined toresult in a desirable lack of perception by a viewer of the transition.

Curves 50 and 52 indicate that, according to one embodiment, a range oftransition profiles may take place such that transitioning from a firstposition to a second position of mirror 20 occurs within the time periodassociated with blue light transmission. FIG. 2A also illustrates thattransitioning back from a second position to a first position alsooccurs during the time period in which the blue light may be displayed.As illustrated, a generally trapezoidal response in which the positionof mirror 20 rises quickly from a first position to a second position isdesirable. In one embodiment, the transition from a first position to asecond position occurs in about 1.3 milliseconds.

FIG. 2B is a graph illustrating a position versus time graph of mirror20 resulting from a step control signal 38 provided by controller 34.Because optical dithering system 14 is a spring mass system, a stepresponse for control signal 38 such as that illustrated in FIG. 2Bresults in transitioning from a first position to a second position witha time versus response curve 42 illustrated in FIG. 2B. Thus, mirror 20overshoots its desired position, swings back, and continues to oscillatefor some time period before it comes to a generally resting position atthe desired second position. This type of response is undesirable, andwould result in a fuzzy image on display 16.

FIG. 2C is analogous to FIG. 2B but illustrates the response of mirror20 in moving from a first position to a second position and then back tothe first position repeatedly over time in response to control signal38. As illustrated, mirror 20 overshoots its desired position and beginsto oscillate until the next period of control signal 38 is provided, inwhich case mirror 20 again overshoots in the opposite direction,resulting in an undesirable response. As described above, controllingthis undesirable response through use of a feedback control system wouldbe prohibitively expensive. Thus, according to the teachings of theinvention, a predetermined control signal is provided that addresses theundesirable response curves for position signal 42 of FIGS. 2B and 2C.

FIG. 3A illustrates a desirable response curve 142 of mirror 20 inmoving from a first position to a second position as well as theassociated control signal 138. Control signal 138 is determined, asdescribed in greater detail below, based upon modeling of the springmass system of optical dithering system 14 and its characteristics arepredetermined independent of the operation of mirror 20. Thus, theposition of mirror 20 at any given time is not utilized to determine thecharacteristics of control signal 138. However, as described in greaterdetail below, the characteristics of predetermined control signal 138may be tuned based upon the position response of mirror 20 in previoustransitions.

As illustrated, desirable response curve 142 quickly rises from a firstposition to a second position with little overshoot and ringing. This isin contrast to undesirable response curve 42 with significant overshootand ringing. Predetermined control signal 138 comprises, in thisexample, an initial step 144 having a pulse width 146 and a magnitude148. Predetermined control signal 138 also includes a quench pulse 150having a width 152 and a magnitude 154 that is equal in magnitude butthe opposite polarity of magnitude 148, in this embodiment. Step width146 of initial step 144 is also referred to herein as quench pulse delay146 because it indicates the delay of quench pulse 152. System modelinghas determined that, according to one embodiment, step width 146 of theinitial step pulse 144 would last for about twenty percent of theresonant response period. The resonant response period herein refers tothe period associated with the natural frequency of the spring masssystem. It has further been determined that the width of quench pulse152 would be about fourteen percent of the resonant response period, inone embodiment. Thus for a 250 Hz system, the quench pulse 150 wouldhave an offset of about 0.8 milliseconds and a hold time of 0.55milliseconds. The ratio between the offset and hold times is dependenton the mechanical Q of the system.

It should be noted that the predetermined control signal 138 is oneexample of a predetermined waveform that provides a desirable response.Other waveform shapes, magnitudes, and frequencies may be utilized basedon modeling of the system to be controlled. As another example, acontrol signal may be provided that is analogous to control signal 138illustrated in FIG. 3A, but in which the magnitude of the control signalafter the quench pulse has a reduced value; however, many other types ofcontrol signals may be used A narrower quench pulse may be used withmore highly-damped or lower Q systems because less energy is required todamp out the overshoot and ringing in such systems.

FIG. 3B is analogous to FIG. 3A but shows the response over timecharacteristics of control signal 138, response curve 142, andundesirable position signal 42.

FIG. 3C is a three-dimensional graph illustrating the overshootamplitudes that occur for signal 142 for various combinations of width152 of quench pulse 150 and width 146 of step pulse 144 (or quench pulsedelay). As illustrated, the minimum overshoot occurs at a combinationcorresponding to the time periods described above. Although theparticular combination of pulse widths may vary according to thecharacteristics of mirror 20, spring 32, and other associatedcomponents, the teachings of the invention recognize that suchcharacteristics may be modeled and a desirable control signal 138 may bedetermined a priori and thus a high bandwidth feedback control system isnot required. However, the teachings of the invention also recognizethat there may be some disparity between the modeled parameters and theactual parameters for any given mirror system 14. Thus, a feedbacksignal 42 is provided to controller 34 to allow fine tuning of widths146 and 152 based on actual system parameters to fine tune thepredetermined control signal. This fine tuning is described inconjunction with FIGS. 4A through 8 below.

FIG. 4A is a block diagram of a system for controlling the positioningof an optical dithering element, such as mirror 20. System 200 includes,in this embodiment, an application specific integrated circuit (ASIC)202, a field programmable gate array (FPGA) 204, a bridge driver 206, atorque motor and mirror 208, and a setpoint position feedback block 210.FPGA 204 may correspond to controller 34 of FIG. 1 and torque motor andmirror block 208 may correspond to voice coil 30 and mirror 20 of FIG.1.

ASIC 202 has a primary purpose of controlling modulation of image source12 to produce light 18. However, field programmable gate array 204receives from ASIC 202 a sub-frame sink signal over line 212 such thatfield programmable gate array 204 may control movement of opticaldithering element 20 such that movements of optical dithering element 20are aligned at an appropriate point in time with respect to thetransmission of light 18, such as is shown in FIG. 2A. A serial bus 214is provided between FPGA 204 and ASIC 202 for writing initial delay andhold time, sample period, and amplitude data to the FPGA and for readingback operational status data from the FPGA. FPGA controller 204'sprimary purpose is to control movement of optical dithering element 20.FPGA controller 204 produces a differential drive signal over lines 216and 218 and provides this signal to bridge driver 206. FPGA controller204 receives feedback over line 222 as described in greater detailbelow. Bridge driver 206 provides the drive signal that drives thetorque motor associated with optical dithering element 20, which in oneexample is voice coil motor 30. Setpoint position feedback block 210represents the measurement of the position of optical dithering element20 and determination of whether the position is higher or lower than adesired setpoint. This indication is provided to FPGA controller 204over line 222. In one embodiment, the sampling rate of the position ofoptical dithering element 20 is about 1000 Hz or four times the naturalresonant frequency of the system.

In operation, FPGA controller 204 provides a predetermined waveform overdifferential pair 216 and 218 to bridge driver 206. Bridge driver 206 inturn drives the torque motor associated with optical dithering elementwith an associated waveform. The resulting position versus time of theoptical dithering element is compared at a plurality of sample points tothe desired setpoint and an error indication of whether the actualposition exceeded or fell below the desired setpoint is provided overline 222 to FPGA controller 204. In response to this feedback, FPGAcontroller 204 modifies the waveform transmitted over lines 216 and 218to compensate for differences between the of the optical ditheringelement at the sample points and the desired setpoint. Additionaldetails of this modification are described below in conjunction withFIG. 4B.

FIG. 4B is a block diagram illustrating portions of FPGA controller 204according to one embodiment of the invention. FPGA controller 204includes a registers and modifiers block 230, a pulse width modulator232, a torque motor waveform generator 234, a sampler 236, and a look-uptable 238.

Registers and modifiers block 230 comprise a plurality of registers forstoring appropriate data signals and associated logic for modifying thedata stored in the registers in response to received feedback. Pulsewidth modular 232 produces a waveform for controlling the magnitude ofthe drive waveform generated by torque motor waveform generator 234.Sampler 236 samples the output of the analog comparator of setpointposition feedback block at predetermined sample intervals. In addition,sampler 236 generates an error signature based upon the sampled signalsas described in detail below and provides the error signature to look-uptable 238. Look-up table 238 determines, based upon the received errorsignal, modifications to the quench pulse delay, the quench pulse width,and the PWM count to produce a more desirable position versus time curveof optical dithering element 20 for subsequent transitions. Thesemodifications are provided as increments and decrements to thepredetermined values corresponding to the predetermined waveform (suchas control signal 138), which are stored in registers and modifiersblock 230. Based on these modifications, revised values are provided byregisters and modifiers block 230 to the pulse width modulator 232 andtorque motor waveform generator 234, as described in greater detailbelow.

Registers and modifiers block 230 produces five signals: a base count240, a pulse width modulation count 242, a quench pulse width 244, aquench pulse delay 246, and a sample period 248. Together base count 240and pulse width modulation count 242 control pulse width modulator 232such that the magnitude of the drive signals on 216 and 218 isappropriate. Quench pulse width signal 244 controls the width of thequench pulse produced at lines 216 and 218. Quench pulse delay signal246 controls the delay of the quench pulse of a drive waveform at lines216 and 218. The output of pulse width modulator 232 is provided overline 250 to torque motor waveform generator 234. The rising edges of theH-bridge drive waveforms on lines 216 and 218 are synchronized to therising edge of PWM signal on line 250. This output is also provided tosampler 236 over line 258, as is a sub-frame sink signal over line 260.At line 222 an indication of whether the position of optical ditheringelement exceeds or falls below a desired setpoint is provided to sampler236.

Sampler 236 takes samples of this high or low signal at appropriate timeperiods. According to one embodiment of the invention, appropriate timeperiods correspond to quadrature points based on the natural frequencyof the inertia/torsion spring system of optical dithering system 14.Thus, according to one embodiment, samples are taken every 90 degrees ofthe natural resonant frequency of optical dithering system 14.Quadrature sampling allows characterization of both the magnitude andphase of position errors. Further, in one embodiment, samples begin atthe end of the quench pulse as indicated by the sample start signal online 262 and the sample interval is set by the sample period on line248. However, sample points may be taken at different times and maybegin at different time periods. According to one embodiment, fivequadrature sample points are taken; however, any suitable number ofsamples may be taken, including two samples. Sampler 236 may incorporatea shift register to store a plurality of sample points and then providea word over line 264 representing an error signature. The word iscomprised of a number of bits equal to the number of sample pointsdesired. In one example, in which five quadrature point samples aretaken, a five-bit word is provided over line 264 that is the errorsignature.

Look-up table 238 is indexed by the error signature word 264. Thus, inthe example where five samples are utilized, look-up table 238 comprises32 entries indexed by 32 different possible five-bit words. An exampleof such a table is described in greater detail in connection with FIG.8. Based upon look-up table 238, increments or decrements are providedover lines 266, 268, and 270 corresponding to increments or decrementsin the stored values associated with quench pulse delay, quench pulsewidth, PWM count. The PWM count determines the magnitude of the quenchpulse. Additional details of the control of optical dithering element 20and look-up table 238 are described in greater detail below inconjunction with FIGS. 5 through 8.

FIG. 4C is a block diagram of an alternative embodiment of a system 300for controlling the dithering of optical element 20. System 300 isanalogous to system 200 except that the functions of FPGA controller 204have been incorporated within application-specific integrated circuit302. Therefore, many of the functions that are executed in firmware ofthe FPGA controller 204 may be executed in the software as described ingreater detail below in conjunction with FIG. 4D.

FIG. 4D illustrates portions of ASIC 302 applicable to the control ofoptical dithering element 20. FIG. 4D is analogous to FIG. 4B exceptthat a software code block 306 is provided. Software code block 306performs similar functions to that of look-up table 238 of FIG. 4B.Software code block 306 receives initial values over line 308 andmodifies these values based upon the error signature 264 in an analogousmanner to that described above in conjunction with FPGA controller 204of FIG. 4B.

FIG. 5 is a graph illustrating a plurality of waveforms associated withcontrol of optical dithering element 20. Waveform 320 is the drivewaveform produced at line 220 for driving the motor that positionsoptical dithering element 20. Waveform 322 represents a setpoint for thedesired position of optical dithering element 20. Note that in thisembodiment, because the position of the mirror of interest occurs onlyin the first period of drive waveform 320, a setpoint is a constantvalue. However, it will be understood that the desired setpoint foroptical dithering element 20 varies between two different positions.Waveform 324 corresponds to the desired position versus time response ofoptical dithering element 20. Waveform 326 corresponds to the undamped,uncontrolled response that would occur for optical dithering element inresponse to a simple step drive waveform for waveform 320 (without thequench pulse). Quadrature sample points 328 illustrate the location atwhich a plurality of quadrature samples of the actual position versustime waveform of optical dithering element 20. Thus, if the sampledvalue falls above setpoint waveform 322 at one of the sample points, anerror indicator is generated indicating a high value at that samplepoint. In one example, a one indicates a high value and a zero indicatesa lower value. It should be understood that if the actual position isequal to the setpoint that either a one or a zero could be provided;however, in one embodiment of the invention if the actual position isequal to the setpoint then a zero is generated.

According to the invention, the sign of the difference between theactual position of mirror 20 and the setpoint 322 at each of the samplepoints 328 is utilized to adjust control signal 320 to reduce overshootand ringing and produce desired waveform 324.

FIG. 6 is a flowchart illustrating one example of a method forcontrolling optical dithering element 20. The method begins at step 402.At step 404 the output of a comparator is sampled at specifiedintervals. In one embodiment, such output could correspond to the outputprovided over line 222 by an analog comparator within setpoint positionfeedback block 210. As described above, the specified intervals mayoccur at the quadrature points corresponding to the natural frequency ofoptical dithering system 14. In one embodiment, the sampling occursafter the end of quench pulse 150 but could also be coincident with theend of the quench pulse 150. At step 406, two or more of the samplescollected at step 404 are combined to form an error signature word.According to one embodiment of the invention five samples are collected;however, two or more samples would also work. At step 408 the binaryword that is created from the two or more samples of step 406 create anerror signal. At step 410, the error signature value is used as apointer to a look-up table, in an embodiment in which a look-up table isused. Further, the value of the error signature may also be used as theargument in a case statement, for example where a software program isutilized for the control function.

At step 412, a quench pulse delay timer, a quench pulse width timer, anda pulse width modulation amplitude control register are incremented,decremented, or held based upon the error signature and the sampleperiod is adjusted. According to one embodiment, the sample period isset to 75 percent of the sum of quench pulse delay and quench pulsewidth. The resulting control signals having the new values are utilizedat step 410 to produce a new control signal. The above procedure may berepeated for each transition of optical dithering element 20, asindicated by reference number 416, or may be repeated as desired toresult in a control signal that generates a desired position versus timesignal of optical dithering element 20. It should be noted that feedbackprocessing and waveform parameter modification can be prioritized tohave a low priority and does not need to occur on each cycle because thecontrol waveform will repeat until the next modification. The methodconcludes at step 418.

According to one embodiment of the invention the increment and decrementvalues are modified as a function of time from turn on of the system.This allows larger step sizes at start up for faster convergence andthen going to smaller step sizes for finer convergence. For example, aseries of timer step sizes of 10, 5, 2 and then 1 and pulse widthmodulation step sizes of 2 and then 1 may be used over the first threeseconds of operation. It should also be noted that the initialquadrature-sampling interval is calculated as a percentage of the sum ofthe initial quench pulse delay and the initial quench pulse width.Modeling systems over a wide range of natural frequencies has shown a 75percent ratio to be optimum. Thus, the time at which sampling pointsoccur is modified as the quench pulse delay and pulse width change tooptimize the drive waveform. If the quadrature-sampling interval isdetermined based upon a percentage of the quench pulse delay and thequench pulse width, this aspect is taken into account and modificationsto the drive waveform produce modifications to the time period forsampling. The incrementing, decrementing or holding of the appropriatevalues to effect modification of the drive waveform may be determined asdescribed in greater detail below in connection with FIGS. 7A through7C.

FIGS. 7A through 7C are graphs illustrating error signatures as afunction of the pulse delay error and pulse width error for drivewaveforms having amplitudes that are low, correct, and high,respectively. Such graphs may be utilized to determine appropriatecorrections to the increment, decrement, or hold rules described above.These graphs were generated by introducing known error in the variousparameters, determining the resulting error signature, and placing theseon these graphs. The error signature maps covered the range of expectederrors. Modification rules for each error signature determined byexamining its occurrence in each error map and deciding the best courseof action to reduce error values and converge to a trapezoidal waveformresponse. These three graphs show error maps for a five-bit sampling.

With reference to FIG. 7A, an error signature having a value of 25indicates that the pulse width is too low and should be incremented,whereas an error signature of 6 indicates that the pulse width is toohigh and should be decremented. An error signature of 25 also indicatesthat the pulse delay is too high and should be decremented. In contrast,an error signature of 6 does not inform whether a pulse delay is corrector incorrect. Thus by mapping the various error signatures and visuallydetermining an appropriate correction, a look-up table, such as thatdescribed below in conjunction with FIG. 8 may be generated. FIGS. 7Band 7C show additional maps for various values of the amplitudeparameters. These three graphs as well as others may be combined togenerate the modification rules illustrated in Table 461 of FIG. 8.

FIG. 8 is a table showing modification rules for a five-bit errorsignature for a particular implementation. It should be noted that theserules may differ from system to system, but may be determined based uponthe teachings described above of modeling a system and determiningappropriate responses due to detected errors. Table 461 comprises asignature error column 462, a delay column 464, a pulse width column 466and a pulse width modulation count 468. As indicated, any givensignature results in a hold, an increment, or a decrement value for eachof the relevant parameters.

Thus, by determining whether the desired waveform exceeds or falls belowa setpoint at a plurality of sample points, the associated controlsignal (or drive waveform may be modified for future transitions ofoptical dithering element 20 to reduce such error. This modification mayoccur without examining the magnitude of the error of the position ofthe optical dithering element 20, but rather by merely examining whetherit is too high or too low. Such a system may be implemented in a muchless costly fashion than a complicated server servo control system.

FIG. 9A is a schematic diagram illustrating measurement of the positionof an optical dithering element 500 according to the teachings of theinvention. As described above, an optical dithering element 500 (such asoptical dithering element 20) may pivot about a point 502 causingrotational movement of optical dithering element 500, as indicated byarrows 504. Rotational movement 504 has an associated translationalmovement 506. For small angles of rotational movement 504, atranslational movement 506 of optical dithering element 500 isessentially in one direction. It is a movement in this direction forwhich feedback is desired for controlling positioning of opticaldithering element 500, as described above. In one embodiment, thesemovements are very small and may be on the order of 0.015 degrees ofrotation and four microns of translation at end 505 of optical ditheringelement 500.

To effect measurement of end 505 of optical dithering element 500 aphotointerrupter 508 is utilized. Photointerrupter 508 includes alight-emitting diode 510, a phototransistor 512, and a slot 513separating light-emitting diode 510 from phototransistor 512. The lighttransmission bundle across slot 513 from the light-emitting diode 510 tothe phototransistor 512 is physically constrained by apertures 514 and515. Disposed within slot 513 is an optical dithering element arm 516,which is also coupled to end 505 of optical dithering element 500.Optical dithering element arm 516 may be formed from any suitablesubstance that may block light transmitted across slot 514. In oneembodiment, optical dithering element arm 516 is a vane made of metal.

According to the teachings of the invention, optical dithering elementarm 516 is positioned slot 513 such that movement 506 of end 505 ofoptical dithering element 500 changes the effective size of apertures514 and 515 by blocking a portion of the light bundle, which results ina change in current through phototransistor 512. The teachings of theinvention recognize that current through phototransistor 512 variesapproximately linearly with respect to the size of the apertures 514 and515 if the current of light-emitting diode 510 is appropriatelycontrolled. Thus, the position of end 505 of optical dithering element500 may be determined as a function of the current throughphototransistor 512. Components illustrated in FIG. 9A for measuring theposition of end 505 of optical dithering element 500 may correspond, inone embodiment, to setpoint position feedback block 210 of FIG. 4A.

Associated control circuitry 518 is provided to appropriately controlthe light-emitting diode current and to generate feedback signalsindicative of the position of end 505 of optical dithering element 500.In one example, these feedback signals include a feedback signal 520,which is an indication of whether the position of end 505 is greater orless than a desired setpoint. In this regard, signal 520 is analogous tosignal 222 of FIG. 4B. Additionally, in one embodiment, a positionsignal indicative of the peak-to-peak swing of the position of end 505may be provided at signal 522. Additional details of control circuitry518 and the operation of photointerrupter 508 are described in greaterdetail below in conjunction with FIGS. 9B through 11.

FIG. 9B is a schematic diagram along lines 9B-9B of FIG. 9A, showingadditional details of apertures 514 and 515. As illustrated in thisview, optical dithering element arm 516 is positioned about halfway downthe height of apertures 514 and 515. Movement along arrows 506 resultsin a small change in the effective size of apertures 514 and 515, asindicated by lines 517. According to one embodiment, the approximatedistance moved by end 505 is about four microns. Thus, according to theteachings of the invention, a fairly small distance change may bemeasured by converting the distance change into current through aphototransistor. This is accomplished by recognizing that thephotocurrent varies approximately linearly with respect to the size ofapertures 514 and 515 if the current through light-emitting diode 510 isappropriately controlled.

FIG. 9C is a graph illustrating current through phototransistor 512versus a height of aperture 514. As illustrated, no current flows whenapertures 514 and 515 are completely blocked and maximum current flowswhen apertures 514 and 515 are completely un-blocked. Three curves areillustrated in FIG. 9C corresponding to three different testedphotointerrupters. These three cases correspond to a high gain unit, amiddle gain unit, and a low gain unit. The high gain unit corresponds toan LED current of approximately 7.04 milliamps required to produce aphotocurrent of 5 amps at maximum aperture. The middle gain unitcorresponds to an LED current of 10.29 milliamps to obtain 5 milliampsof photocurrent at maximum aperture. The low gain unit corresponds to anLED current of 14.7 milliamps in order to reach a photocurrent of 5milliamps at maximum aperture. As illustrated in FIG. 9C, photocurrentvaries linearly with the size of aperture 460 over a range of aperturesizes. In particular, near the middle of the aperture size, photocurrentvaries very linearly with aperture size. Thus, disposing opticaldithering element arm 516 such that it blocks approximately half ofapertures 514 and 515 can result in a linear change in photocurrent inresponse to small changes in the position of optical dithering elementarm 516 from its setpoint.

FIG. 10 is a circuit diagram illustrating additional details ofphotointerrupter 508 and control circuitry 518. As illustrated,photointerrupter 508 includes a light-emitting diode 510 and aphototransistor 512 separated by a slot 513. Disposed within slot 513 isan optical dithering element control arm 516 coupled to opticaldithering element 500. In one embodiment, optical dithering element 500is a mirror. According to one embodiment of the invention, a setpointcurrent through phototransistor 512 is set to be 2.5 milliamps, whichcorresponds to a particularly linear region of the photocurrent versusaperture size graph of FIG. 9C. It is changes in current about thesetpoint that are indicative of the position of end 505 of mirror 500.

Setting of the phototransistor current setpoint may be achieved throughuse of a voltage source 521 and associated resistor 518. According toone embodiment, voltage source 521 is 10 volts and resistor 518 is 2kilo-ohms, and thus a current of 2.5 milliamps through phototransistor512 produces a voltage of 5 volts at node 523.

A voltage at node 523 is provided to one input of an operationalamplifier 517 and compared to a second input, which receives anoperating point bias. In this example, the operating point bias is 5volts. Thus, when the current through phototransistor 512 is 2.5milliamps, the output of operational amplifier 517 is also at 5 voltscorresponding to no position change of end 505 of mirror 500.

Operational amplifier 517 produces an output at node 525. The output ofnode 525 is provided to an input of comparator 524. Comparator 524compares a voltage indicative of the position of end 505 of mirror 500to a setpoint for a desired position of end 505 of mirror 500. In oneexample, the setpoint bias provided to the other input of comparator 524is set to 5.5 volts. The value of resistor 527 is selected such that thevoltage at node 525 varies from the setpoint of 5 volts by approximatelyplus or minus 0.5 volts. In one example, the resistance of resistor 527is 27 kiloohms.

Thus, according to the teachings of the invention, linear movement ofend 505 of mirror 500 may be detected by converting this linear movementinto a change in the effective size of apertures 514 and 515 of theassociated photointerrupter 508 and then translating the resultingchange in photocurrent into a voltage indicative of the position changeof end 505 of mirror 500. The teachings of the invention recognize thatthe characteristics of the photocurrent versus aperture size curve, suchas FIG. 9C, may vary depending on the LED current through LED 510. Thus,the current through LED 510 must be controlled such that thephotocurrent versus aperture size maintains a known linear relationshipsuch as that illustrated in FIG. 9C. This figure corresponds to aparticular example in which 5 milliamps of current is achieved withmaximum aperture size. But as described above, such photocurrentcorresponds to varying levels of LED current based upon the gain of thephotointerrupter. Thus, a controlled system is provided that maintainsthe LED current at a level that corresponds to 5 milliamps at themaximum aperture. This roughly corresponds, in this example, toachieving an average voltage level of 5 volts at node 525. Thus, anintegrator 519, which receives the operating point bias of 5 volts, hasas one input the voltage at node 525. The voltage at node 525 isintegrated over time to provide a feedback signal to LED 510. Thus, ifthe setpoint is off, for example, case in which the setpointphotocurrent is 3 milliamps, then the voltage at node 525 is too high.This causes integrator 519 to integrate down such that it lowers LEDcurrent through LED 510. This lowering of the current brings thephotocurrent back to its setpoint of 2.5 milliamps.

FIG. 10 illustrates additional circuitry that may be utilized, ifdesired, for additional reasons. A synchronous rectifier and low passfilter 528 is coupled at node 525 to provide an indication of theaverage peak-to-peak amplitude at node 525. This output is provided toone input of the comparator 530 and compared to the other input, whichis a second setpoint bias. The resulting output of comparator 530indicates whether the voltage representing the average peak-to-peakmirror movement at 505 is above or below the second setpoint bias.

FIG. 11 is a flow chart illustrating a method 600 for providing aposition indication of an optical dithering element. The method beginsat step 602. At step 604 a photointerrupter having associatedphototransistor and light-emitting diodes separated by an slot isprovided. At a step 606 an optical dithering element arm coupled to anoptical dithering element is disposed at a setpoint within the apertureof the photointerrupter. The setpoint is approximately mid way between afully open and a fully closed aperture, such that a change in aperturesize at the setpoint results in an approximate linear change in thecurrent through the phototransistor. At step 508, the current resultingin a change of position of the optical dithering element arm in responsethe optical dithering element is measured. During this process, thecurrent through the LED is controlled to maintain a desired setpoint ofthe phototransistor, and thus, the phototransistor has a relativelyconstant proportional gain with respect to changes in effective aperturesize, and therefore position of the optical dithering element. At step12 the signal is generated that is indicative of the position of theoptical dithering element. The method concludes at step 614.

Although the present invention has been described in detail, it shouldbe understood that the various changes, substitutions, and alterationscould be made hereto without departing from the spirit and scope of theinvention as defined by the appended claim.

1-2. (canceled)
 3. A method of increasing a perceived resolution of adisplay comprising: directing light at an optical dithering element;repeatedly transitioning the optical dithering element from a firstposition to a second position and then back to the first position suchthat the mirror alternately reflects light to a first position on thedisplay and then to a second position on the display; and wherein eachtransition of the mirror comprises controlling any overshoot or ringingin the position of the optical dithering element by providing apredetermined control signal to the optical dithering element.
 4. Themethod of claim 3, wherein the predetermined control signal comprises aquench pulse.
 5. The method of claim 3, wherein the predeterminedcontrol signal is dependent on a position versus time response of theoptical dithering element of a previous transition.
 6. The method ofclaim 3, wherein the optical dithering element has a spring systemassociated therewith and wherein the predetermined control signal isdependent on a natural frequency of the combination of the opticaldithering element and the spring system.
 7. The method of claim 3,wherein the optical dithering element has a spring system associatedtherewith and wherein the predetermined control signal is dependent on aQ of the combination of the optical dithering element and the springsystem.
 8. The method of claim 3 wherein the predetermined controlsignal is determined by simulation of the position over time of theoptical dithering element.
 9. The method of claim 3, wherein directinglight comprises directing light by a digital micro-mirror imagingdevice.
 10. The method of claim 3, wherein directing light comprisesdirecting different colors of light at different time periods and whenrepeatedly transitioning the optical dithering element comprisestransitioning the optical dithering element when no color other thanblue is directed at the optical dithering element.
 11. The method ofclaim 3 wherein the position versus time response of the opticaldithering element for a transition from the first position to the secondposition and then a transition from the second position to the firstposition is generally trapezoidal.
 12. The method of claim 4, whereinthe width of the quench pulse is based at least in part on a positionversus time response of the optical dithering element of a priortransition.
 13. The method of claim 4, wherein the magnitude of a delayassociated with the quench pulse is based at least in part on a positionversus time response of the optical dithering element of a priortransition.
 14. The method of claim 3, wherein a response period isdefined as the time period between the beginning of a transition fromthe first position to the second position and the beginning of the nextsubsequent transition from the second position to the first position;and wherein the predetermined control signal comprises: a step pulsehaving a first magnitude and having a width of approximately twentypercent of the resonant response period; and a quench pulse immediatelyfollowing the step pulse, the quench pulse having a magnitude equal tothe first magnitude but opposite in direction and having a width ofapproximately fourteen percent of the resonant response period.
 15. Themethod of claim 3, and further comprising automatically adjusting thepredetermined control signal in response to any overshoot that occursduring transition between the first and second positions of the opticaldithering element during previous transitions.
 16. The method of claim15, wherein automatically adjusting comprises increasing an amplitude ofthe predetermined control signal to produce a desired position versustime response for the optical dithering element and then decreasing theamplitude to maintain the desired position versus time response for theoptical dithering element. 17-28. (canceled)
 29. A method of controllinga spring-mass system comprising: transitioning the mass from a firstposition to a second position; and wherein transitioning the masscomprises controlling any overshoot or ringing in the position of themass by providing a predetermined control signal to the mass.